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SubjectRe: [PATCH] usb: hub: Wait for connection to be reestablished after port reset
On Thu, 1 Dec 2016, Guenter Roeck wrote:

> On a system with a defective USB device connected to an USB hub,
> an endless sequence of port connect events was observed. The sequence
> of events as observed is as follows:
>
> - Port reports connected event (port status=USB_PORT_STAT_CONNECTION).
> - Event handler debounces port and resets it by calling hub_port_reset().
> - hub_port_reset() calls hub_port_wait_reset() to wait for the reset
> to complete.
> - The reset completes, but USB_PORT_STAT_CONNECTION is not immediately
> set in the port status register.
> - hub_port_wait_reset() returns -ENOTCONN.
> - Port initialization sequence is aborted.
> - A few milliseconds later, the port again reports a connected event,
> and the sequence repeats.
>
> This continues either forever or, randomly, stops if the connection
> is already re-established when the port status is read. It results in
> a high rate of udev events. This in turn destabilizes userspace since
> the above sequence holds the device mutex pretty much continuously
> and prevents userspace from actually reading the device status.
>
> To prevent the problem from happening, let's wait for the connection
> to be re-established after a port reset. If the device was actually
> disconnected, the code will still return an error, but it will do so
> only after the long reset timeout.
>
> Cc: Douglas Anderson <dianders@chromium.org>
> Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Acked-by: Alan Stern <stern@rowland.harvard.edu>

> ---
> drivers/usb/core/hub.c | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
> index cbb146736f57..9bcb649e0e8c 100644
> --- a/drivers/usb/core/hub.c
> +++ b/drivers/usb/core/hub.c
> @@ -2731,8 +2731,15 @@ static int hub_port_wait_reset(struct usb_hub *hub, int port1,
> if (ret < 0)
> return ret;
>
> - /* The port state is unknown until the reset completes. */
> - if (!(portstatus & USB_PORT_STAT_RESET))
> + /*
> + * The port state is unknown until the reset completes.
> + *
> + * On top of that, some chips may require additional time
> + * to re-establish a connection after the reset is complete,
> + * so also wait for the connection to be re-established.
> + */
> + if (!(portstatus & USB_PORT_STAT_RESET) &&
> + (portstatus & USB_PORT_STAT_CONNECTION))
> break;
>
> /* switch to the long delay after two short delay failures */
>

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